Solid oxide fuel cells (SOFC's), like other fuel cells, are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. Desirably, power generation systems incorporating high-temperature fuel cells have the potential for higher efficiencies and power outputs. Exemplary high-temperature fuel cells have operating temperatures above about 600° C.; and SOFC's typically operate in a range between about 600° C. and 850 C. A fuel cell produces electricity by catalyzing fuel and oxidant into ionized atomic hydrogen and oxygen at, respectively, the anode and cathode. The electrons removed from hydrogen in the ionization process at the anode are conducted to the cathode where they ionize the oxygen.
In the case of a solid oxide fuel cell, the oxygen ions are conducted through the electrolyte where they combine with ionized hydrogen to form water as a waste product and complete the process. The electrolyte is otherwise impermeable to both fuel and oxidant, and merely conducts oxygen ions. This series of electrochemical reactions is the sole means of generating electric power within the fuel cell. It is therefore desirable to reduce or eliminate any mixing of the reactants. Otherwise, a different result might occur, such as combustion, which does not produce electric power, and therefore reduces the efficiency of the fuel cell.
A typical fuel cell operates at a potential of less than about one (1) Volt. To achieve sufficient voltages for power generation applications, a number of individual fuel cells are integrated into a larger component. To create a fuel stack, an interconnecting member or “interconnect” is used to connect the adjacent fuel cells together in an electrical series, in such a way that the fuel and oxidants of the adjacent cells do not mix together. Interconnects play a crucial role in an SOFC stack. They must exhibit excellent electrical conductivity, as well as strength and thermal and dimensional stability at elevated temperatures. The interconnects should also be impervious to oxygen and hydrogen, so as to prevent their contact during cell operation. They should also be relatively easy to fabricate, according to configurations that allow for efficient cell assembly, as well as the flow of air and fuel through the cell.
SOFC stacks for planar cells very often rely on internal manifold channels for the transport of fuel and air to the active region of the cells, via one or more interconnects. The pathway and distribution of these gaseous reactants within the manifolds and interconnects has been the subject of a great deal of research over many years, since it has such a critical effect on stack operation. Reference is made, for example, to “Flow Distribution Analysis of the Solid Oxide Fuel Cell Stack under Electrical Load Conditions, J. Jewulski et al, B07-SOFC; Cells and Components 3, 2009. In addition to ensuring proper gas flow through the channels in the cell, the interconnect structure must effectively provide seals around the manifold openings—seals that can withstand high-temperature operation of the cell.
In a typical SOFC interconnect design, a parallel-flow of fuel and air (often a counter-flow arrangement) is preferred for optimum performance of the fuel cell, due in part to the large amount of heat arising from fuel reformation and energy production. However, a counter-flow arrangement usually requires that fuel and air ports (manifold openings) be next to each other. Their adjacent location can, in some cases, undesirably reduce access of the manifold to the flow area of the cell, and can also increase the possibility of fuel-to-air leaks.
In contrast, a cross-flow design, i.e., where fuel flow and airflow intersect at 90°, is often advantageous, in that the manifold openings can be spaced from each other, e.g., on opposing sides of a planar interconnect. This allows for ease-of-manufacture, and minimizing the possibility of premature contact between air and fuel. However, the cross-flow design is sometimes disadvantageous for other reasons, such as the tendency to develop large temperature gradients during fuel cell operation.
With these general considerations in mind, improved interconnects for solid oxide fuel cells would be welcome in the art. Interconnect configurations that enhance the flow of fuel and air through the fuel cell structure would be especially desirable, perhaps combining the advantages of cross-flow design and counter-flow (parallel) designs. The interconnects should also be capable of providing sealing structures around any manifold opening or other passageway where air- and fuel flow must be tightly controlled or prevented. Furthermore, the interconnects should be relatively easy to fabricate, and to assemble into a full SOFC power stack on an industrial scale.